In operation of valves for controlling fluid flow or pressure drop, cavitation within the valve has always been a matter of serious concern. Where excessive cavitation occured, throttling valves have been literally destroyed within a few hours of operation. In general, cavitation resulted during throttling of a liquid under pressure when the pressure drop of the liquid in the region of the throttling orifices falls below the vapor presure of the liquid. In the typical situation, where water was the control fluid, this caused the momentary formation of steam bubbles just downstream of the throttling surfaces. As the flowing liquid continued beyond the throttling surfaces, the its pressure rose to exceed the vapor pressure of the liquid, causing a sudden collapse or implosion of the steam bubbles. This resulted in a sonic shock wave, which was transmitted through the liquid to the adjacent surfaces of the valve. Where the energy of the shock waves exceeded the fatigue limits of the valve material, the material was quickly cavitated away, and the valve was destroyed.
In order to minimize cavitation damage it has been proposed heretofore to effect the desired pressure drop in a series of individual steps, so calculated that the pressure drop in any individual step was sufficiently low that cavitation minimized. Other proposals involved dividing the flowing fluid into a large plurality of individual streams, so that the cavitation energy of each stream was reduced to an acceptable level. Although these prior proposals made significant improvements in the operating life of high pressure throttling valves, cavitation remained a serious problem.
Valve noise has been traditionally associated with large pressure drops across a valve. In the past, low noise valves have been constructed so as to sequentially restrict pressure drop or with an additional diffuser downstream of the valve's internal restriction or with a muffler attached to the outlet piping of a valve. According to accepted principles, a given fluid flowing at a given rate through a valve which experiences a pressure drop will produce significantly less noise if the given pressure drop occurs over successive restrictions as distinguished from occurring across a single restriction of a size required to produce the same pressure drop. The restrictions may be in the form of perforations, apertures, or fluid passages acting in parallel. The total restriction present in such a valve for any given plug position is thus the sum of the individual sequential of restrictions or stages.